1. Field of the Invention
The present invention relates to a Fresnel lens sheet for use in a rear projection type television, especially a Fresnel lens sheet suitable for condensing light obliquely projected from a projector (light source) placed at its rear to let the light emerge toward the viewer's side as nearly parallel rays. In addition, the present invention relates to a process of producing the Fresnel lens sheet, to a mold for use in this production process, and to a rear projection screen comprising the Fresnel lens sheet.
2. Description of the Related Art
There has conventionally been known, as a large-size-screen television, a rear projection type television in which light, imaging light, is projected on a rear projection screen from a projector placed at the rear of the rear projection screen to display an image to viewers.
In such a rear projection type television, light projected from a projector is spread and the spread light is projected on a rear projection screen, so that it is necessary to keep a certain distance between the projector and the rear projection screen. A television of this type therefore has had the problem that the space in the direction of depth is apt to be large.
To solve this problem, there has been proposed a rear projection type television in which a projector 30 is, as shown in FIG. 17, placed obliquely below a rear projection screen 21′ at its rear, and light S is projected on the rear projection screen 21′ obliquely and upwardly from this projector 30 in order to make the space in the direction of depth smaller.
A rear projection screen comprising a total reflection Fresnel lens that can condense light projected obliquely from its rear (Japanese Laid-Open Patent Publication No. 208041/1986) is conveniently used as the rear projection screen 21′ for use in the rear projection type television as shown in FIG. 17. The total reflection Fresnel lens herein denotes a lens having a plurality of prisms, where projected light is refracted at the first plane (plane of refraction) of each prism and then totally reflected at the second plane (plane of total reflection) of each prism to emerge toward the viewer's side.
In a rear projection screen comprising such a total reflection Fresnel lens, the light path of projected light is corrected by totally reflecting the projected light. Therefore, the rear projection screen can attain high transmittance even when projected light is obliquely incident on the rear projection screen at a large angle.
In a Fresnel lens sheet having such a total reflection Fresnel lens, since light is obliquely projected on the Fresnel lens sheet as described above, the angle of incidence at which projected light is incident on the Fresnel lens sheet is usually in the order of 35 to 75°. A resin material with a refractive index of approximately 1.45 to 1.65 is usually used as a molding material for the Fresnel lens sheet. In this case, each prism on the Fresnel lens sheet has an apical angle α of approximately 30 to 40°, an angle of the plane of refraction γ of approximately 78 to 90°, and an angle of the plane of total reflection β of approximately 65 to 50° (where α+β+γ=180°) (see FIG. 18).
In such a Fresnel lens sheet, the angle δ of a root between two neighboring prisms is nearly equal to the apical angle α of each prism. Therefore, also the apical angle ω of an angular protrusion created between two neighboring molding grooves C in a mold D for use in the molding of the Fresnel lens sheet is equal to the apical angle α of each prism and is as considerably sharp as approximately 30 to 40° (see FIG. 18). For this reason, the tips of the angular protrusions of the mold D easily bend and also often get curved toward the counter-light-source side as, for example, shown in FIG. 19, so that the problems as described below used to occur.
In this Specification, “the light-source side of a mold” means, in a mold for use in the molding of a Fresnel lens sheet, the side with which a portion of the Fresnel lens sheet to be situated on the side close to a light source upon use of the Fresnel lens sheet is molded, while “the counter-light-source side of a mold” means the side with which a portion of the Fresnel lens sheet to be situated on the side distant from the light source upon use of the Fresnel lens sheet is molded. More specifically, if each prism on a Fresnel lens sheet that will be obtained by molding using the mold is considered, “the light-source side of a mold” refers to the side with which the plane of refraction of each prism is molded, while “the counter-light-source side of a mold” refers to the side with which the plane of total reflection of each prism is molded. In FIG. 19, the plane Ca, on the lower side in the figure, of each molding groove C in the mold D forms the plane of refraction of each prism, and the plane Cb, on the upper side in the figure, of the molding groove C in the mold D forms the plane of total reflection of each prism, so that the lower side in the figure is the light-source side of the mold D and that the upper side in the figure is the counter-light-source side of the mold D.
The problems that occur when a Fresnel lens sheet is molded by the use of the mold D shown in FIG. 19 will be described hereinafter.
(1) Problems in the Production of Fresnel Lens Sheet
In the molding of a Fresnel lens sheet using the mold D shown in FIG. 19, after loading a resin in the mold D and curing this resin, the Fresnel lens sheet 11, molded product, is released from the mold D. At this time, since the tips of the angular protrusions of the mold D cut into the Fresnel lens sheet 11, it has been difficult to release the Fresnel lens sheet 11 from the mold D (see FIG. 20A). Moreover, in this condition, if it is tried to release the Fresnel lens sheet 11 from the mold D by force, the Fresnel lens sheet 11 has been damaged sometimes.
(2) Problems upon Use of Fresnel Lens Sheet
The curves of the angular protrusions of the mold D are transferred to the Fresnel lens sheet 11 when the Fresnel lens sheet 11 is molded by the use of the mold D shown in FIG. 19, so that each root 15 between two neighboring prisms 12 among a plurality of the prisms 12 situated on the incident side of a base 11a gets curved as shown in FIG. 20B. When light S is obliquely projected on the Fresnel lens sheet 11 in which the roots 15 between each two neighboring prisms 12 are curved in this way, part of light totally reflected from the plane of total reflection 14 of each prism 12 is reflected again at the curved root 15 (a curved portion of the plane of refraction 13 of the root 15) to become stray light Y. The stray light Y thus produced is reflected at the plane of emergence 11b and then returns to the incident side via the base 11a, and this light is refracted or reflected again at the plane of refraction 13 or the plane of total reflection 14 of each prism 12 to emerge in late from the position on the plane of emergence 11b that is different from the position of normal emergence. This causes such troubles as the occurrence of double image.